This project will continue studies of the mechanisms that lead to antigen-specific tolerance of mature CD8 plus T cells in the periphery, focusing on the roles of activation-induced cell death and non- responsiveness. In comparison to CD4 plus cells, relatively little is known about these mechanisms in CD8 plus T cells. The Specific Aims include: (Aim 1). To further characterize activation-induced anergy and death in CD8 plus T cells in vitro. Experiments will be done to test two hypothesis: (I) Induction of non-responsiveness in CD8 plus T cells following an initial response to antigen depends upon the nature of the primary costimulatory ligands (B7 versus ICAM-1) and (ii) The balance of death and anergy following response of CD8 plus cells to antigen depends upon the quantitative "strength" and duration of the initial signals (antigen dose/duration, costimulatory ligand density, etc). (Aim 2) To determine the mechanisms of in vitro induction of anergy and death in CD8 T cells. Responses to B7 and ICAM-1 costimulatory ligands will be further examined to determine the expression and roles of Bcl-xL and Bcl-2 survival proteins, the role of CTLA-4 in induction of death and non-responsiveness, and the roles of Fas/Fas-L and TNF-alpha in apoptotic death. (Aim 3) To define the requirements for in vivo induction of activation -induced anergy and death in CD8 T cells. An adoptive transfer system has been established for studying the responses of CD8 plus cells from TCR transgenic mice that allows that number, locations and phenotype of the cells to be determined during the course of an in vivo response. Predictions made by the in vitro studies under Aims 1 and 2 will be tested in vivo using microspheres having well defined antigen and costimulatory ligand compositions as "artificial APCs". Results obtained in this project studying mature CD8 plus T cells will complement those obtained in the other Program projects examining similar issues in CD4 plus cells, and class I restricted thymocytes.